1. Field of the Invention
The present invention relates to a wind turbine generator, a control method for a wind turbine generator, a wind turbine generator system, and a control method for a wind turbine generator system.
2. Description of Related Art
Generally, an electric-power supply system is required to maintain the balance in supply and demand of the amount of electric power, the voltage, and the frequency at constant levels. Conventionally, in wind turbine generator, power output has been controlled by detecting the power output, rotational speed of the wind-turbine rotor, and so on and feeding back the detection results. However, control that reduces the frequency and the voltage fluctuation in the utility grid has not been performed. Therefore, by supplying the power output from the wind turbine generator independently of the state of the utility grid (supply and demand of electric power, load factor, frequency, voltage, etc.), there is a risk that the utility grid will be unstable.
Therefore, to connect the wind turbine generator to the utility grid, it is necessary to conform with a grid code, which sets the stability of voltage and frequency, the supply stability of reactive power, and the readiness in the event of failure. For example, even when the frequency of the utility grid fluctuates with reference to a predetermined rated frequency, when the fluctuation is within a predetermined time (for example, 10 seconds) and smaller than or equal to a predetermined value (for example, 5% of a reference frequency) set by the grid code, it is necessary to continue the operation of the wind turbine generator without disconnecting it.
Furthermore, when the utility grid becomes unstable, there is a demand for operation that is not only has no disconnection, but also more actively contributes to the stabilization of the utility grid. Accordingly, as a technique of controlling the power output of the wind turbine generator taking the status of the utility grid into account, Japanese Translation of PCT International Application, Publication No. 2003-535561 discloses a technique in which the power output of the wind turbine generator is dropped when the frequency of the utility grid rises.